Ballast for discharge devices



Oct. 16, 1962 J. G. SOLA 3,059,143

BALLAST FOR DISCHARGE DEVICES Filed May 9, 1960 INV EN TOR.

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Unite States 3,059,143 BALLAST FOR DISCHARGE DEVICEQ Joseph G. Sola, River Forest, 111., assignor to Basic Products Corporation, West Milwaukee, Wis., a corporation of Wisconsin Filed May 9, 1960, Ser. No. 27,817 9 Claims. (Cl. 315-138) This application is a continuation-in-part of an application of Joseph G. Sola, Serial No. 4,477, filed January 25, 1960, entitled Ballast for Discharge Devices, now abandoned.

This invention relates to an alternating current power supply apparatus including a high reactance transformer and capacitor to be connected between a source of voltage and one or more negative resistance characteristic discharge devices for starting and operating thereof, wherein the transformer output circuit, whether the secondary winding alone in the case of an isolating transformer or the combined secondary and primary windings in the case of an auto-transformer, and the capacitor in series are connected in series with the discharge devices, and it is an object of the invention to provide improved apparatus of this nature.

Apparatus of the nature referred to, commonly termed ballasts, wherein the primary winding of the transformer is adapted for connection to a voltage source of suitable frequency and magnitude, and the output of the transformer in series with the capacitor is adapted to be connected in series circuit with the negative resistance characteristic discharge device is well known, being disclosed for example in the Patents 2,870,398, dated January 20, 1959; 2,858,479, dated October 28, 1959; and 2,346,621, dated April 11, 1941; all to the same inventor as the subject application.

Ballasts of the nature referred to, so far as the particular practical applications are concerned, fall into two categories, those for supplying discharge devices of high internal pressure and those for supplying discharge devices of low internal pressure. Thus, by way of example only and not by way of limitation, high internal pressure discharge devices may be of the mercury vapor type for outdoor lighting, and printing lamp applications, and discharge devices of the low internal pressure type may be of the familiar fluorescent lamp type for interior illumination. Both fields of application are highly competitive and the fluorescent field is essentially one of high volume. Accordingly, costs of manufacture are very important and it is an object of the invention to provide ballasts of the nature referred to at lower costs than predecessor ballasts for the same application without impairing the performance thereof. Experience has shown that better performance may be achieved with ballasts according to the invention for the same application than with predecessor ballasts and this, accordingly, constitutes a still further object of the invention.

Ballasts disclosed in the patents referred to, operate the discharge devices to which they are connected at regulated wattages which in many instances may be substantially constant wattage, at proper values of crest factor (peak to R.M.S. ratio) of the operating current of the ballast and at high input power factor, these qualities being maintained over a certain range of variation of the source or primary voltage. Such ballasts as manufactured operate at high flux density, i.e., saturation, in the secondary portion of the transformer core to achieve these desired advantages over the range of variation of the source voltage. While there is no known analytical relationship between the saturation of the secondary core portion and the qualities of substantially constant or regulated wattages, good current crest factor and high 3,059,143 Patented Oct. 16, 1962 input power factor in ballasts of the character indicated and, while individual ones of these qualities may be achieved in other ways, they were achieved by applicants ballasts which had a high degree of saturation in the secondary core portion.

it is a further object of the invention to provide im proved ballasts of the character indicated wherein a saturation flux density condition exists in the secondary portion of the transformer core, with no non-magnetic gap therein, which will provide the desired value of crest factor in the discharge device current irrespective of whether the ballast transformer is connected as an isolation transformer or as an auto-transformer.

The saturation flux density in the secondary portion of the transformer core was achieved in the prior ballasts through the use of the capacitor alone as it cooperates with the reactance of the transformer. Consequently, a high voltage appeared across the capacitor during operation. Moreover in ballasts according to Patents 2,870,- 398 and 2,858,479, that is, ballasts for supplying relatively high pressure metallic vapor discharge devices, the voltage across the capacitor immediately after the discharge device struck was higher than during steady state or stable operation.

Accordingly, it is a further object of the invention to provide improved apparatus of the character indicated wherein the operating voltage of the capacitor is reduced over values needed for known apparatus, thereby making it possible to achieve reductions in construction costs.

It has been found that the necessary high flux density in the secondary portion of the core can be achieved in part, according to the invention, by reducing the crosssectional area of the secondary portion of the core. The capacitor still cooperates with the transformer to produce the desired high flux density, but a substantial proportion of the high flux density is achieved by the reduction in the secondary core cross-section.

The reduction in cross-sectional area of the secondary core portion results in a core arrangement which lends itself to a scrapless construction during manufacture and makes additional space available for the secondary winding, thereby enabling changes to be made in the secondary winding, such, for example, as changes in coil dimensions, which result in further reductions in weight and cost of the finished product, and it is therefore a further object of the invention to provide improved devices of the character indicated which are efficient in operation and economical to manufacture.

It is a further object of the invention to provide apparatus of the character indicated, which has improved starting and operating characteristics.

It is a further object of the invention to provide improved apparatus of the character indicated for fluorescent lamp discharge devices which produces greater light output from the discharge devices than heretofore and at lower costs of manufacture.

Further and additional objects and advantages will become apparent as the description proceeds.

For a better understanding of the invention, reference should be had to the accompanying drawing, in which FIGURE 1 is a plan view partially in section of one form of transformer, which may be utilized in carrying out the invention;

FIG. 2 is a circuit diagram of apparatus embodying the invention and showing the electrical connections of the transformer illustrated in FIG. 1, together with other circuit components;

FIG. 3 is a plan view partially in section of another form of transformer which may be utilized in carrying out the invention;

FIG. 4 is a circuit diagram of apparatus embodying the Invention and showing the electrical connections of the transformer illustrated in FIG. 3, together with other circuit components;

FIG. is a drawing of an oscillogram showing the output voltage of the transformer of FIGS. 3 and 4, under starting conditions of the discharge devices;

FIG. 6 is a drawing of an oscillogram showing the voltage for the same windings as FIG. 5 under operating conditions of the discharge devices; and

FIG. 7 is a drawing of an oscillogram showing the current through the discharge devices of FIG.4 under operating conditions.

Referring to FIGURE 1 of the drawing, one embodiment of the invention is shown comprising, according to the invention, the combination of a transformer 10, and a capacitor 11, supplying a negative resistance characteristic discharge device 12, from a source of voltage S, connectable to terminals 13.

The transformer 10 is shown including a primary winding 14, a secondary winding and an iron core 16, the iron core being of the shell type having a central leg 17 and an outer shell consisting of side legs 18 and 19, and end legs 21 and 22. In the form illustrated, the side legs 18 and 19, and the end legs 21 and 22 are integral with each other, although other forms of construction may be used. The core may be formed of laminations punched from suitable transformer sheet steel in the form of the outer shell comp-rising the legs 18, 19, 21 and 22, the laminations forming the central leg 17 being punched as part of the same operation. A suitable number of laminations are assembled together to give a stack of desired thickness.

The windings 14 and 15 may be form wound and disposed on the central leg 17, the resulting assembly then being pressed into the assembled stack of outer laminations.

Disposed between the primary windings 14 and 15, there are high reluctance shunts consisting of portions 23 and 24, formed integrally with side legs 18 and 19 respectively, and portions 25 and 26 formed integrally with the central leg 17. The portions 23 and 25 are separated by a non-magnetic gap shown as an air gap 27, and the portions 24 and 26 likewise, are separated by a nonmagnetic gap shown as an air gap 28. The high reluctance shunts 23, 25, 27, and 24, 26, 28, provide a leakage flux path whereby portions of the fluxes of the primary and secondary windings may link the respective one of the windings to the exclusion of the other one. In this manner, the core provides a high leakage reactance for each of the primary and secondary windings, although other well-known constructions for obtaining high leakage reactance may be utilized.

The central leg 17 is divided into two portions, a portion 29 upon which the primary winding 14 is disposed and a portion 31 upon which the secondary winding is disposed. In accordance with the invention, and as may be seen more clearly in 'FIGURE 1, the cross-sectional area of the secondary portion 31 is substantially less than the cross-sectional area of the primary section 29.

As shown, the width of each of side legs 18 and 19, and end legs 21 and 22, is greater than one-half the width of primary portion 29 of the central leg. Accordingly, the total cross-sectional area of outer legs 18 and 19 is greater than the cross-sectional area of primary portion 29. Similarly for the total cross-sectional area of end legs 21 and 22. Since the cross-sectional area of secondary portion 31 is less than the cross-sectional area of primary portion 29, the cross-sectional area of secondary portion 31 is even more less than the total crosssectional area of the outer or side legs 18 and 19. The larger total cross-sectional area of the outer legs results in a lower flux density in these members and conse quently results in reduced extraneous or stray fields surrounding the device during operation. Where these advantages are not needed, the cross-sectional areas of the central leg and the outside legs may be the same. Constructions utilizing wide outer legs are shown and claimed in Patent No. 2,806,199 to the same inventor.

For a further description of the invention, and for a better understanding of the operation thereof, reference should be had to FIG. 2, which is a circuit diagram showing the circuit connections of the various components. In FIG. 2, the reference characters are the same as those used in FIGURE 1, to designate corresponding parts. The core 16 and the high reluctance shunts 23, 25, 27, and 24, 26, 28 are shown schematically and the primary and secondary windings 14 and 15, respectively, are shown disposed on different sides of the shunts. The primary portion 29 of the core is schematically shown to be of greater cross-sectional area than the secondary portion 31 of the core.

The operating circuit of the secondary winding, according to the invention, is a series circuit and may be traced as follows: From one terminal 30 of the secondary winding to one terminal of capacitor 11, from the other terminal of capacitor 11 to one terminal 33 of discharge device 12, and from the other terminal 32 thereof to the other terminal of secondary winding 15. As shown, the secondary winding 15 having terminals 30 and 32 is connected as an isolation transformer. When the primary Winding 14 is connected to a source S of suitable voltage and frequency, for example, volts at 6 0 cycles, the open circuit secondary voltage developed as a result of the turns ratio of the secondary winding 15 and primary winding 14 is sufiicient to strike the discharge device 12. At the instant prior to striking, there being no current flow, there is no voltage drop across capacitor 11 and thus, the full voltage developed by secondary winding 15 appears across discharge device 12.

In one practical form of construction, according to the invention, the primary winding 14 had 96 turns and the secondary winding 15 had 1612 turns, thereby developing an open circuit voltage of 1700 volts at 115 primary volts, which secondary voltage appeared across the terminals 32 and 33 of discharge device 12, and was sufficient to strike it. The discharge device 12 utilized in this construction was designated as UA24B by the manufacturer thereof, intended for photoprinting purposes, and was rated at 1200 watts, 1.2 amperes. The striking voltage of this device is approximately 1700 volts, and the operating voltage is about 1200 volts although these values may vary somewhat with diiferent lamps of the same type.

It is characteristic of discharge devices of the type here involved, that the striking voltage is higher than the operating voltage, the difference between these two 'voltages being absorbed by the ballast which comprises the inductive reactance of the secondary 15 and the capacitive reactance of the capacitor 11. This phenomenon is well known.

Also, in the well-known types of ballasts, such as those disclosed in the patents referred to hereinbefore, when operating with photoprinting lamps or street lighting lamps for example, there is a warm-up period for the lamp preceding the steady state or operating period. During the warm-up period the voltage across the capacitor 11 decreases from the higher value at striking to a lower value at operating or steady state conditions. Inasmuch as the capacitor 11 has to be selected to also Withstand the voltage after striking and during the warm-up period, it is necessary to provide a capacitor of higher voltage rating than the operating voltage. This is uneconomical because of the higher cost of higher voltage rated capacitors for the same volt-ampere rating in the voltage range here involved, and is therefore an incentive for reducing the capacitor voltage under any and all conditions of operation. The microfarad value of the capacitor is selected in ballasts according to the invention as well as in prior ballasts to provide rated output, i.e., light or wattage (and current) to the discharge device, the core and coils having been selected. Experience indicates the general characteristics of the core and coils and it has been found that a suitable core and coils may on short circuit of the secondary winding give a short circuit current at a primary voltage in the middle of the voltage range, approximately the rated current of the discharge device in the case of high pressure devices.

The core and coils form a high reactance transformer and the air or non-magnetic gap of the shunt is one factor in selecting the said short circuit current. There are of course other factors such as the cross-section of the iron core on which the coils are disposed and the numbers of turns in the coils.

The non-magnetic gap is also one of the factors together with high degree of saturation of the iron in the secondary portion of the core involved in determining the degree of wattage or light regulation of the discharge device as the primary voltage varies over its predetermined range. As the non-magnetic gap is made longer more energy may be transferred to the load and poorer regulation may result, and as the non-magnetic gap is made shorter, less energy may be transferred to the load and better regulation may result, although other variations are possible.

In ballasts of the nature here involved, whether according to the invention or according to prior devices, the final design is a compromise of various factors such as turns, core dimensions, non-magnetic gap, capacitor, etc. and such ballasts provide substantially constant or at least a regulated wattage to the discharge device.

According to the invention, if the cross-sectional area of the secondary portion 31 of the central leg 17 is reduced appropriately relative to the crosssectional area of the primary portion 29 of the central leg, the voltage across the capacitor 11 just after striking can be substantially reduced. Also the capacitor voltage under steady state conditions can be substantially reduced to be about equal to or even less than the open circuit voltage of secondary winding 15. This advantage can be achieved without foregoing other advantages of prior devices such as high power factor at the input terminals and substantially constant wattage to the discharge device.

In the practical construction referred to for 115 primary volts, the voltage across capacitor 11 just after striking was about 2050 volts and it came down, after a period of several minutes, to a steady state or normal operating voltage of about 1685 volts, which is about equal to the 1700 volt open circuit voltage of secondary winding 15. Just after striking the voltage across secondary winding 15 was about 2050 volts and during the warmup period it raised slightly to about 2060 volts at steady state operating conditions.

By way of comparison the voltages of a predecessor device for operating the UA24B lamp, may be given, which predecessor device utilized the same core as the said practical construction, except that the primary and secondary core portions corresponding to portions 29 and 31 had the same cross-sectional area as primary portion 29, and the air gaps corresponding to air gaps 27 and 28 each had a length of 0.090 inch as compared with 0.130. The primary and secondary windings of the two devices were the same. In the practical construction, the capacitor 11 had a capacity value of 1.845 microfarads whereas the corresponding capacitor in the predecessor device had a capacity value of 1.59 microfarads, the difference in capacity value being necessary to provide the necessary current, 1.38 amps for the predecessor device and 1.27 amps for the device according to the invention, for supplying rated wattage to the lamp in each case. In the said predecessor device for 115 primary volts, the open circuit voltage of the secondary winding was about 1,750 volts, the voltage across the capacitor just after striking was about 2,470 volts and it came down, after a period of several minutes, to a steady state or normal operating voltage of about 2,100 volts. The voltage across the secondary winding Was about 2,470 volts just after strik- 6 ing and it raised slightly to about 2,490 volts at steady state operating conditions.

Thus it may be observed that the device according to the invention had a voltage of about 415 volts less across capacitor 11 under operating conditions and a voltage of about 420 volts less just after striking than the predecessor device, and a corresponding voltage difierence existed between the secondary Winding voltages of the two devices under similar conditions. The power factor of the predecessor device was percent and of the device according to the invention was 97.2 percent.

Unlimited reduction of the cross-sectional area of the secondary portion 31 of the central leg relative to the primary cross-sectional area is not possible since such reduction ultimately results in unduly increased primary exciting currents with consequent undesirable reductions in power factor at the input terminals 13. Similarly, insuificient wattage ultimately may be supplied to the discharge device 12 if the reduction in secondary crosssectional area is too great. The amount of reduction of secondary cross-sectional area also depends on the type of discharge lamp used, the amount of capacitor voltage reduction desired, and variations in other factors such as the magnetic quality of the transformer steel. It has been found that good results were achieved in the practical construction described wherein the decrease in crosssectional area was such that the voltage across capacitor 11 during operation was about equal to the open circuit voltage of the secondary winding.

In ballasts of the natureinvolved in this application, that is, ballasts including a high reactance transformer and a capacitor wherein the secondary winding of the transformer is connected in series with the capacitor and this series combination is connected in series with the lamp or other discharge device, it is necesary for the core portion upon which the secondary winding is disposed to operate at a high flux density, that is, a flux density such that a condition of substantial saturation exists in the said core portion. The needed secondary high flux density is higher than the flux density of the portion of the core upon which the primary winding is disposed because to make the primary flux density of the same magnitude would result in unduly large primary exciting currents. In this manner, the secondary winding and the capacitor supply regulated voltage and regulated wattage to the discharge device load. The prior art devices such as those referred to herein provide substantially constant wattage and appropriately conforming voltage to the lamp discharge device over an input or primary voltage range of about twelve percent above and twelve percent below a nominal value, such as volts, and throughout such range of primary voltage provide high input power factor, that is, above 90 percent.

In these prior devices, the necessary high flux density in the secondary Winding portion of the core was brought about by the effect of the capacitor upon the inductive reactance of the secondary winding because the cross-sec tional area of the secondary winding portion of the core was the same as that of the primary winding portion.

According to the invention, the secondary core portion 31 has a reduced cross-sectional area of a certain amount, the reduction being such that the core portion 31 has a flux density greater than the flux density of core portion 29. Dependent upon the flux density of core portion 29, the decrease in area of core portion 31 is such as to produce a condition of substantial saturation in core portion 31. Thus in the practical device referred to, the flux density of core portion 29 was about 106,000 lines per square inch and the flux density in core portion 31 on open circuit was about 117,200 lines per square inch, the crosssectional area of the core portion 31 being twenty percent less than that of core portion 29.

The flux density of 117,200 lines per square inch is a condition of saturation as is Well known. By using the capacitor 11, the flux density in core portion 31 is built up to 142,000 lines per square inch, this flux density serving in combination with the complete structure to produce constant wattage in device 12 over the range of primary voltage variations, from 100 volts to 130* volts, the power factor varying from 99.5 percent at 100 volts to 88.3 percent at 130 volts and being 97.2 percent at 115 volts.

By way of contrast, the predecessor device referred to, under corresponding conditions and assumptions, had a flux density of 106,000 lines per square inch in the primary winding core portions, and had a flux density of 96,600 lines per square inch in the secondary Winding core portion on open circuit, the loss being due to the presence of the shunt. Under steady state operating conditions the secondary win-ding core portion had a flux density or": 137,500 lines per square inch, a high degree of saturation, produced by the capacitor alone. This predecessor device delivered constant wattage to the discharge device over the same primary voltage range and had an input power factor ranging from 100 percent at 100 volts to 97.0 percent at 130 volts, being 100 percent at 115 volts.

Flux densities as given in this specification are computed uSing the formula V=4.44 BANF, wherein V is volts across the winding, B is the flux density in lines per square inch, A is the cross-sectional area in square inches, N is the number of turns in the winding and F is the frequency in cycles per second. The values computed using this formula are not exact because resistance drops in the primary winding have been ignored and it has been assumed that all flux is in the core and that the flux wave is a sine wave, but the values are of proper order of magnitude and are satisfactory for the present purpose, including comparison of devices upon which similar measurements have been made.

The transformer utilized in the practical construction referred to had a three and three-eighths inch stack of laminations of 26 gauge M-19 grade, hot rolled transformer steel. During operation, with 115 volts supplied to the primary winding, the current through the lamp was 1.27 amperes and the crest factor, that is the ratio of peak current to R.M.S. current, was 1.86. The microfarad value of capacitor 11 was somewhat higher than the microfarad value of the corresponding capacitor in the said predecessor circuit for the same discharge device, but the current peak to R.M.S. ratio was improved substantially, for example, from 2.02 to 1.86, which is an additional advantage. In the improved construction, the drop-out voltage of the lamp was at least as good as that of prior devices.

In this same practical construction, the width of primary portion 29 of the central leg was 1.25 inches, whereas the width of the secondary portion 31 of the central leg was 1.0 inch. That is to say, the reduction in cross-sectional area of the secondary portion of the central leg was about twenty percent. The length of the air gap 27 was 0.13 inch as was the air gap 28.

The secondary winding does not fill the space provided because of the reduction in cross sectional area of core portion 31.

Referring now to FIGS. 3 and 4 of the drawing, a further embodiment of the invention is shown comprising, in combination a transformer 40 and a capacitor 41, supplying a pair of negative resistance characteristic discharge devices 42 and 43, from the source of voltage S connected to terminals 44.

The transformer 45 is shown including a primary winding 45, a secondary winding 46, and an iron core 47, the iron core being of the shell type having a central leg 43, and outer legs 49 and 51. The primary winding 45 and the secondary winding 46 are connected in auto-transformer relationship by means of conductor 52, and the output of winding 45 and winding 46 is connected across discharge lamps 42 and 43, through capacitor 41. The discharge devices 42 and '43 may be, for example, fluorescent lamps of the continuously heated filament type, one

current form of which is known as the rapid start lamp, the lamps being started in sequence by virtue of the shunting capacitor 53, and operated in series. Other forms of lamps or negative resistance characteristic dis-- charge devices and starting arrangements may, of course, be used without departing from the spirit and scope of the invention.

While the transformer of FIGS. 3 and 4 is connected in auto-transformer relationship and a pair of discharge devices in series are started and operated by the trans former, the principles of operation are the same as those set forth in connection with the embodiment of the invention illustrated in FIGS. 1 and 2.

The iron core 47 consisting of central leg 48 and outer legs 49 and 51, preferably may be of the scrapless lamination type. To achieve this, a series of central legs 48 are punched from the same strip of steel, successive ones of the said legs being punched as mirror images of each other. The outer legs 49 and 51 are also punched from a single strip of transformer steel and single ones of these legs are punched successively as mirror images of each other.

.Thus the outer leg 49 consists of a primary portion 54, a secondary portion 55 and a secondary end portion 56, and the outer leg 51 consists of a primary portion 57, a secondary portion 58, a secondary end portion 59. The primary portion 54 and the secondary portion 55 may be considered to terminate at juncture 61 and the primary portion .57 and the secondary portion 58 may be considered to terminate at juncture 62. The length of secondary portion 55 from the interior edge 63 of secondary end portion 56 to the juncture 61, is substantially the same as the length of primary portion 54 from the juncture 61 to the end edge 64 of primary portion 54. Similarly, the length of secondary portion 58 from the juncture 62 to the interior edge 65 of secondary end portion 59, is substantially the same as the length of primary portion 57 from the juncture 62 to the end edge 66 of primary portion 57. The length of the junctures 61 and 62, between the interior edges of the secondary portions 55 and 55 to the interior edges of primary portions 54 and 57 are the same. Accordingly, it may be visualized that either of the outer legs 49 and 51 may be turned over to interfit with the other one to form an essentially scrap-less lamination, so far as these two legs are concerned.

The central leg 48 consists of a primary portion 67 and a secondary portion 68 which may be considered to have their juncture at edges 69. The transverse dimension of the secondary portion 58 is less than the transverse dimension of primary portion 67 to provide a reduction in cross-sectional area of the secondary portion 63, in accordance with the invention. The Width of secondary portion 68 is such that in cooperation with the secondary end portions 56 and 59, a window space is defined in cooperation with secondary portions 55 and 58 to receive the secondary winding 46. The left-hand end of primary portion 6'7 terminates in a pair of members '71 and '72 of such dimension that when taken in cooperation with the primary portions 54 and 57, a window space is defined for the primary winding 45. The length of secondary portion 65 from edges 69 to the right-hand edge is essentially the same as the length from edges 69 to the inner edges '73 and '74 of members 71 and 72, respectively, so that successively punched ones of central lamination 48 may be mirror images of each other in the punching operation. The widths of secondary portions 55 and 58 are greater than the respective primary portions 54 and 57 and the edges er and 62 are disposed relative to the edges 69, whereby shunts including the non-magnetic, for example, air gaps 75 and 76, are obtained. By virtue of the shunts, including the air gaps 75 and 76, transformer 40 is a high leakage reactance transformer.

A suitable number of outer leg laminations 55 and 58 and a corresponding number of central leg laminations 48 are assembled together to give a stack of desired thickness, the legs being held together by any means desired.

The windings 45 and 46 may be form wound and disposed on the central leg 48 as shown. The various dimensions may be chosen as desired so long as the secondary portion 68 of the central leg 48 is reduced in crosssectional area as compared to the primary portion 67 of the central leg in the amount needed for the particular application. In a practical core construction actually made and operated, the Width of the primary portions 54 and 57 are each about equal to one-half of the width of the primary portion 67 and the widths of secondary end portions 56 and 59 are each equal to about one-half of the width of the secondary portion 68. Other appropriate dimensions will be given subsequently in this specification.

For a further description of the invention, and for a better understanding of the operation thereof, reference should be had to FIG. 4, which is a circuit diagram corresponding to circuit diagram of FIG. 3. The core 47 and the high reluctance shunt including the nonmagnetic gaps 75 and 76 are shown schematically and the primary and secondary windings 45 and 46, respectively, are shown disposed on opposite sides of the shunt. The primary portion 67 of the core is schematically shown to be of greater dimension than the cross-section 48 of the core.

The output circuit of the transformer, according to the invention, is a series circuit and may be traced as follows: From one terminal 77 of the secondary winding t0.one terminal 78 of the capacitor 41, from the other terminal '79 of the capacitor to the terminal 81 of one filament of lamp 43, through lamp 43 to a common terminal 80 of lamps 43 and 42, through lamp 42 to the other terminal 82 of lamp 42, through conductors 83 and 84 to primary winding 45 and through primary winding 45 and conductor 52 to the remaining terminal of secondary winding 46.

Since the lamps 42 and 43 as illustrated are of the continuously heated filament type, filament heating windings 85, 86 and 87 are provided in close coupled relationship with primary winding 45, the filament heating winding 85 being connected by means of conductors 88 and 83 to the left-hand filament of lamp 42, the filament heating winding 87 being connected by means of conductors 89, 91 and 92, to the right-hand filament of lamp 43 and filament heating winding 86 being connected by means of conductors 93, 94 and 95, to the remaining respective filaments in parallel of the lamps 42 and 43.

As shown, the pair of lamps 42 and 43 are connected in a sequence starting and series operating arrangement and thus as has already been pointed out, a starting impedance comprising the capacitor 53 is connected across the filaments of lamp 43. The capacitor 53 is connected as shown to conductors 89 and 91 at one end of lamp 43 and to conductors 93 and 94 at the other end.

The total output voltage of windings 45 and 46 is made sufficient by means of the voltage source S and the turns ratio of winding 46 relative to winding 45, so that suflicient open circuit voltage is available to strike one of the lamps 42 and 43. It is sufiicient to strike lamp 42 in the circuit as shown, inasmuch as the capacitor 53 at the instant of starting, by-passes the lamp 43. After lamp 42 strikes the ionization current flowing through starting capacitor 53 develops sufficient voltage across lamp 43 to strike this lamp, whereby both lamps are struck and connected in series across primary windings 45 and 46 in series with capacitor 41. The described system of sequence starting and series operating discharge devices is well known.

When the primary winding 45 is connected to the source S of suitable voltage and frequency, for example 1 18 volts at 60 cycles, the open circuit voltage developed as described is suflicient to strike and operate the discharge devices. At the first instant of striking there is no voltage drop across capacitor 41, since there is no current flow as 10 is well understood. The filament heating windings 85, 86 and 87 being energized from the instant of energization of primary Winding 45, supply heating currents to the filaments of lamp 42 and 43, and accordingly, reduce the ionization voltage of these devices to make it easier to strike them.

The capacitor 41 is selected, as has been described, with a given set of core and coils to produce rated light output of the discharge devices. The core and coil form a high reactance transformer and as a general guide it may be noted that the short circuit current of the secondary winding, when the primary is supplied with middle range rated voltage may be about equal to twice rated current of the fluorescent lamps. The general principles of constructing ballasts for fluorescent lamps is the same as stated for the apparatus of FIGS. 1 and 2.

In one practical form of construction already referred to, as shown according to the invention in FIGS. 3 and 4, the primary winding 45 had 720 turns of No. 27 copper wire and the secondary winding 46 had 1440 turns of No. 28 copper wire, whereby an open circuit voltage of 175 volts was developed across secondary winding 46 at 118 volts. Accordingly, an output circuit voltage across conductor 84- 'and terminal 77 of 290 volts was provided, which output voltage appeared across terminals and 82 of lamp 42, and was sulficient to strike it. The current thus flowing through capacitor 53 which in the device described was .05 microfar-ad, developed a voltage of 278 volts R.M.S. which was sufficient to strike lamp 43. The lamps 42 and 43 were thus struck, the lamps used in the practical construction being designated as rapid start lamps by the manufacturer thereof, intended for illumination purposes and rated at 40 watts each and .43 ampere. The rated striking voltage of the lamps is no less than 205 volts R.M.S. and the operating voltage is about 102 volts.

When both lamps 42 and '43 are operated in the circuit as described in the practical construction, the voltage across capacitor 41 for 118 primary volts was 252 volts R.M.S. An oscillogram of this voltage, while not shown in the drawings, showed it to be substantially a sine wave having a crest factor of about 1.437. The lamp current was about 0.39 ampere having a peak value of 0.604 ampere and a crest factor of 1.570, the wave form of the lamp current being as shown in FIG. 7.

In a predecessor device, for operating the sametwo fluorescent lamps, having the same series starting and Sequence operating arrangement, as well as the filament heating windings, and having a transformer in which the central core leg had the same cross-sectional area throughout the primary winding and secondary winding portions, but having a slot within the secondary portion of the central leg of the core, the voltage developed across the capacitor corresponding to capacitor 41 was 307 volts R.M.S. of substantially sine wave. The lamp current for such device was 0.403 ampere R.M.S. having a peak value of about 0.69 ampere and a crest factor of 1.67. The general configuration of the current wave was similar to that in FIG. 7.

Thus according to the invention, reducing the crosssectional area of secondary portion 68 of the central leg of the transformer core as compared to the cross-sectional area of the primary portion 67 and producing the saturation flux density in the secondary portion 68 during operating conditions but without the presence of any air gap or slot, results in a reduction of the voltage across capacitor 41 and a decrease in the R.M.S. value of the current through it, thereby efiecting a net saving in the capacitor for the voltage range here involved.

The open circuit voltage, that is, the combined voltage of the primary and secondary windings for the device according to the invention, was 290 volts R.M.S. and the corresponding voltage for the predecessor device was 289 volts, and thus it may be observed that the effect of the reduction in cross-sectional area of the secondary portion of the central leg of the transformer is to reduce the oper- 1 I :ating voltage of the capacitor 41, relative to open circuit voltage of the transformer windings. Referring to FIG. 5, the wave shape of the striking voltage of the device according to the invention may be observed. The voltage as shown in FIG. 5 had an R.M.S. value of about 290 volts, a peak of about 475 volts and a crest factor of about 1.650. After both lamps 42 and 43 have struck in such device, the combined voltage of windings 45 and 46 had a value of about 30 7 volts R.M.S. with a peak value of about 521 volts and a crest factor of 1.70, the wave shape according to an oscillograrn being shown in FIG. 6.

The predecessor device correspondingly had an R.M.S. operating voltage across its output windings of about 330 volts, a peak value of about 560 volts and a crest factor of about 1.695. The voltage wave shape having substantially the same general form as shown in FIG. 6. The open circuit voltage of 289 volts had a peak value of about 454 volts and a crest factor of 1.590. The crest factor of the lamp current during operation which is an important factor according to the lamp manufacturer, was 1.57 for the device according to the invention and 1.67 according to the predecessor device. Thus as may be observed, by the values given, the device according to the invention on open circuit had a slightly higher crest factor of voltage that is to say, a slightly greater peaking of the voltage wave which is an asset in starting the lamps, and after both lamps have struck the crest factor of the operating current is lower in the device according to the invention than the predecessor device which is a further advantage. Accordingly, the reduction in cross-sectional area of the central leg produces a double advantage, namely, better starting and better operation.

By way of comparison, the predecessor device referred to herein had a primary winding of 710 turns of No. 27 wire and secondary coil of 1287 turns of No. 27 /2 wire. The central leg of the core had a width of .812 inch, a stack thickness of inch, and the cross-sectional area thereof was uniform throughout its length except for a bridged gap in its secondary portion. The overall dimensions of the core were 4.406 inches in length and 2.125 inches in width. The capacitor corresponding to 41 in the predecessor device had a value of 3.75 microfarads, the capacitor corresponding to 53 had a value of .05 microfarad and the gap corresponding to nonmagnetic 75 was .075 inch.

Similarly, in devices according to the invention, the overall dimensions of the core were 3.844 inches in length and 2.125 inches in width. The stack thickness was inch, the width of primary portion 67 was inch and the width of secondary portion 68 was 7 inch. The nonmagnetic gaps 75 and 76 each had a length of .125 inch. The capacitor 41 had a value of 4.0 microfarads and the capacitor 5 3 had a value of .05 microfarad. Thus, it can be seen that substantial savings in the amount of iron and copper were achieved.

For corresponding conditions of operation of the predecessor device and the device according to the invention for operating the same lamps and with 118 volts applied to the primary winding of the transformer, the primary power factor was 99.2% for the device in accordance with the invention and 97.5% for the predecessor device, and according to an arbitrary scale of measurement the light output of the device according to the invention was 70.8 and the light output of the predecessor device was 69.5. Over the permissible range of primary voltage of 106 volts to 130* volts, the primary power factor of the device according to the invention varied from 99.9% to 97% and the light output varied from 66.0 to 75, and the predecessor device had a power factor varying from 99.1% to 96.6%, whereas the light output varied from 64.7 to 73.5. Thus it may be seen that the primary power factor of the two devices, while not greatly different from ecah other, was nevertheless better in the device according to the invention and the light output likewise was better in the device according to the invention.

The current trend in fluorescent lamp specifications is toward higher light outputs. It can be seen that the improved light output of the device according to the invention is thus a distinct advantage.

Unlimited reduction of the cross-sectional area of the secondary portion 68 of the central leg relative to the cross-sectional area of the primary portion 67 of the central leg is not possible for reasons already pointed out in connection with the embodiment of FIG. 1. Ultimately such increased reduction in the secondary cross-sectional area results in high primary winding exciting currents with concomitant reduction in input power factor. Similarly, insufiicient wattages, and therefore insufiicient light output, may be obtained from the fluorescent lamps if the crosssectional area reductions are too great. In the devices according to the invention it has been found that the reduction in cross-sectional area can be such that the voltage across the series capacitor 53 after operation is substantially less than the open circuit voltage of the combined primary and secondary windings.

The variations in light output with primary voltage variations as indicated reflect some variations in wattage supplied to the lamp for these variations in primary voltage. The variations, however, are reasonably small and the wattage may be deemed to be regulated.

In the device according to the invention and computed by the formulae and subject to the assumption already stated and at 118 volts primary, the flux density in the primary portion 67 of the central leg was 95,000 lines per square inch and the flux density in the secondary portion ,68 of the central leg on open circuit was 120,000 lines per square inch, which under operating conditions increased to 138,000 lines per square inch.

By way of contrast the predecessor device under corresponding conditions had a flux density of 92,500 lines per square inch in the primary portion of the core, and a flux density of 75,800 lines per square inch in the secondary portion of the core upon open circuit which under operating conditions increased to a flux density of 101,000 lines per square inch.

The transformer core of the practical construction referred to had laminations of 24 gauge M-22 cold reduced steel, which is the same steel used in the predecessor device.

Filament heating windings 85 and 87 had 23 turns of No. 27 copper wire and filament heating winding 86 had 26 turns of No. 27 wire.

Resistors 96 and 97 of the order of magnitude of 1 megohm are connected across capacitors 41 and 53 to drain off any charges which may exist thereon when the system is de-energized.

While particular embodiments of the invention have been shown, it will be understood, of course, that the invention is not limited thereto since many modifications may be made, and it is, therefore, contemplated by the appended claims to cover any such modifications as fall within the true spirit and scope of the invention.

The invention having thus been described, what is claimed and desired to be secured by Letters Patent is:

l. Alternating current power supply means for starting and operating negative resistance characteristic discharge means inoluding at least one negative resistance characteristic discharge device from a source of predetermined voltage and frequency comprising, a transformer having a core of magnetic material including a winding leg having first and second portions, a primary winding on said first portion and adapted for connection to said source, a secondary winding on said second portion adapted to be connected in circuit with such negative resistance characteristic discharge means each of whose devices has predetermined starting and operating voltages, and a high reluctance magnetic shunt magnetically disposed between said primary and secondary windings and also between said first and said second portions of said winding leg, said primary and said secondary windings being connected in voltage additive auto-transformer rela tionship to form an output circuit and the ratio of turns of said primary Winding and said secondary winding being such that the open circuit voltage of said output circuit when said primary winding is energized from said source is sufiicient to start such at least one discharge device of such discharge means, said second portion of said winding leg having a reduced cross-sectional area relative to the cross-sectional area of said first portion of said Winding leg, and a capacitor connected to one terminal of said output circuit and being adapted to be connected to one terminal of such discharge means, the other terminal of said output circuit being adapted to be connected to the other terminal of such discharge means thereby defining a series operating circuit comprising said output circuit, said capacitor and such discharge means when connected in circuit, said capacitor having a capacity value such as to provide substantial-1y rated output in such discharge means. 7

2. Alternating current power supply means for starting and operating negative resistance characteristic discharge means including at least one negative resistance characteristic discharge device from a source of predetermined voltage and frequency comprising, a high reactance transformer having a core of magnetic material including a winding leg having first and second portions, a primary winding on said first portion and adapted for connection to said source, a secondary winding on said second portion adapted to be connected in circuit with such negative resistance characteristic discharge means each of whose devices has predetermined starting and operating voltages, said primary and said second windings being connected in voltage additive auto-transformer relationship to form an output circuit and the ratio of turns of said primary winding and said secondary winding being such that'the open circuit voltage of said output circuit when said primary winding is energized from said source is sufficient to start such at least one discharge device of such discharge means, said second portion of said winding leg having a reduced cross-sectional area relative to the cross-sectional area of said first portion of said winding leg, and a capacitor connected to one terminal of said output circuit and being adapted to be connected to one terminal of such discharge means, the other terminal of said output circuit being adapted to be connected to the other terminal of such discharge means thereby defining a series operating circuit comprising said output circuit, said capacitor and such discharge means when connected in circuit, said capacitor having a capacity value such as to provide substantially rated output in such discharge means, the reduction in cross-sectional area of said second portion of said Winding leg being no greater than that whereby the remaining cross-sectional area serves to retain high input power factor during steady state operating conditions.

3. Alternating current power supply means for starting and operating negative resistance characteristic discharge means including at least one negative resistance characteristic discharge device from a source of predetermined voltage and frequency comprising a transformer having a core of magnetic material including a winding leg having first and second portions, a primary winding on said first portion and adapted for connection to said source, a secondary Winding on said second portion adapted to be connected in circuit with such negative resistance characteristic discharge means each of Whose devices has predetermined starting and operating voltages, and a high reluctance magnetic shunt magnetically disposed between said primary and secondary windings and also between said first and said second portions of said winding core leg, said primary and said secondary windings being connected in voltage additive auto-transformer relationship to form an output circuit and the ratio of turns of said primary Winding and said secondary winding being such that the open circuit voltage of said output circuit when said primary winding is energized from said source is sufficient to start such at least one discharge device of such discharge means, said second portion of said winding leg having a reduced cross-sectional area relative to the crosssectional area of said first portion of said winding leg, and a capacitor connected to one terminal of said output circuit and being adapted to be connected to one terminal of such discharge means, the other terminal of said output circuit being adapted to be connected to the other terminal of said discharge means thereby defining a series operating circuit comprising said output circuit, said capacitor and such discharge means when connected in circuit, said capacitor having a capacity value such as to provide substantially rated output in such discharge means during operation, the reduction in cross-sectional area of said second portion of said Winding leg being such that the voltage across said capacitor during steady state operating conditions of such discharge means is not substantially greater than the said open circuit voltage of said output circuit.

4. Alternating current power supply means for starting in sequence and operating in series a pair of negative resistance characteristic discharge devices from a source of predetermined voltage and frequency comprising, a transformer having a core of magnetic material including a winding leg having first and second portions, a primary winding on said first portion and adapted for connection to said source, a secondary winding on said second portion adapted to be connected in circuit with such pair of negative resistance characteristic discharge devices, each of which has predetermined starting and operating voltages, and a high reluctance magnetic shunt magnetically disposed between said primary and secondary windings and also between said firs-t and said second portions of said winding leg, said primary and said secondary windings being connected in voltage additive auto-transformer relationship to form an output circuit and the ratio of turns of said primary winding and said secondary winding being such that the open circuit voltage of said output circuit when said primary winding is energized from said source is suliicient to start one of such discharge devices, said second portion of said winding leg having a reduced cross-sectional area relative to the cross-sectional area of said first portion of said winding leg, and a capacitor connected to one terminal of said output circuit and being adapted to be connected to one end terminal of one of such discharge devices, the other terminal of said output circuit being adapted to be connected to one end terminal of the other of such discharge devices, the other end terminals of each of such discharge devices being connected together, thereby defining a series operating circuit comprising said output circuit, said capacitor and such pair of discharge devices when connected in circuit, said capacitor having a capacity value such as to provide substantially rated output in such pair of discharge devices, and a starting impedance connected across the end terminals of one of such discharge devices.

5. Alternating current power supply means for starting in sequence and operating in series a pair of negative resistance characteristic discharge devices from a source of predetermined voltage and frequency comprising, a high reactance transformer having a core of magnetic material including a winding leg having first and second portions, a primary winding on said first portion and adapted for connection to said source, and a secondary winding on said second portion adapted to be connected in circuit with such pair of negative resistance characteristic discharge devices, each of which has predetermined starting and operating voltages, said primary and said secondary windings being connected in voltage additive auto-transformer relationship to form an output circuit and the ratio of turns of said primary winding and said secondary Winding being such that the open circuit voltage of said output circuit when said primary winding is energized from said source is sufiicient to start one of such dis- 15 charge devices, said second portion of said winding leg having a reduced cross-sectional area relative to the crosssectional area of said first portion of said winding leg, and a capacitor connected to one terminal of said output circuit and being adapted to be connected to one end terminal of one of such discharge devices, the other terminal of said output circuit being adapted to be connected to one end terminal of the other of such discharge devices, the other end terminals of each of such discharge devices being connected together, thereby defining a series operating circuit comprising said output circuit, said capacitor and such pair of discharge devices when connected in circuit, said capacitor having a capacity value such as to provide substantially rated output in such pair of discharge devices, the reduction in cross-sectional area of said second portion of said winding leg being no greater than that whereby the remaining cross-sectional area serves to retain high input power factor during steady state operating conditions, and a starting impedance connected across the end terminals of one of such discharge devices.

6. Alternating current power supply means for starting in sequence and operating in series a pair of negative resistance characteristic discharge devices from a source of predetermined voltage and frequency comprising, a transformer having a core of magnetic material including a winding leg having first and second portions, a primary winding on said first portion and adapted for connection to said source, a secondary winding on said second portion adapted to be connected in circuit with such pair of negative resistance characteristic discharge devices, each of which has predetermined starting and operating voltages, and a high reluctance magnetic shunt magnetically disposed between said primary and secondary windings and also between said first and said second portions of said winding leg, said primary and said secondary windings being connected in voltage additive auto-transformer relationship to form an output circuit and the ratio of turns of said primary winding and said secondary winding being such that the open circuit voltage of said output circuit when said primary winding is energized from said source is sufficient to start one of such discharge devices, said second portion of said winding leg having a reduced cross-sectional area relative to the cross-sectional area of said first portion of said winding leg, and a capacitor con nected to one terminal of said output circuit and being adapted to be connected to one end terminal of one of such discharge devices, the other terminal of said output circuit being adapted to be connected to one end terminal of the other of such discharge devices, the other end terminals of each of such discharge devices being connected together, thereby defining a series operating circuit comprising said output circuit, said capacitor and such pair of discharge devices when connected in circuit, said capacitor having a capacity value such as to provide substantially rated output in such pair of devices, the reduction in cross-sectional area of said second portion of said winding leg being such that the voltage across said capacitor during steady state operating conditions of such pair of discharge devices is not substantially greater than the said open circuit voltage of said output circuit, and a starting impedance connected across the end terminals of one of such discharge devices.

7. Alternating current power supply means for starting in sequence and operating in series a pair of fluorescent discharge lamps from a source of predetermined voltage and frequency comprising, a high reactance transformer having a core of magnetic material including a winding leg having first and second portions, a primary winding on said first portion and adapted for connection to said source, and a secondary winding on said second portion adapted to be connected in circuit with such pair of fiuorescent lamps, each of which has predetermined starting and operating voltages, said primary and said secondary windings being connected in voltage additive auto- 16 transformer relationship to form an output circuit and the ratio of turns of said primary winding and said secondary winding being such that the open circuit voltage of said output circuit when said primary winding is energized from said source is sufiicient to start one of such lamps, said second portion of said winding leg having a reduced cross-sectional area relative to the cross-sectional area of said first portion of said winding leg, and a capacitor connected to one terminal of said output circuit and being adapted to be connected to one end terminal of one of such lamps, the other terminal of said output circuit being adapted to be connected to one end terminal of the other of such lamps, the other end terminals of each of such lamps being connected together, thereby defining a series operating circuit comprising said output circuit, said capacitor and such pair of lamps when connected in circuit, said capacitor having a capacity value such as to provide substantially rated output in such pair of lamps, the reduction in cross-sectional area of said second portion of said winding leg being such that the voltage across said capacitor during steady state operating conditions of such pair of lamps is not substantially greater than said open circuit voltage of said output circuit, and a starting impedance connected across one of such lamps.

8. Alternating current power supply means for starting in sequence and operating in series a pair of fluorescent discharge lamps from a source of predetermined Voltage and frequency comprising, a transformer having a core of magnetic material including a central leg having first and second portions and two outer legs, one each of which is disposed on each side of said central leg, a primary winding on said first portion of said central leg and adapted for connection to said source, a secondary winding on said second portion of said central leg adapted to be connected in circuit with such pair of fluorescent lamps, each of which has predetermined starting and operating voltages, and a high reluctance magnetic shunt magnetically disposed between said primary and secondary windings and also between said first and second portions of said central core leg, said primary and said secondary windings being connected in voltage additive auto-transformer relationship to form an output circuit and the ratio of turns of said primary winding and said secondary winding being such that the open circuit voltage of said output circuit when said prim-ary winding is energized from said source is sulficient to start one of such lamps, said second portion of said central core leg having a reduced cross-sectional area relative to the crosssectional area of said first portion of said central leg, and a capacitor connected to one terminal of said output circuit and being adapted to be connected to one end terminal of one of such lamps, the other terminal of said output circuit being adapted to be connected to one end terminal of the other of said lamps, the other end terminals of each of such lamps being connected together, thereby defining a series operating circuit comprising said output circuit, said capacitor and such pair of lamps when connected in circuit, said capacitor having a capacity value such as to provide substantially rated output in such pair of lamps, the reduction in cross-sectional area of said second portion of said central leg being such that the voltage across said capacitor during steady state operating conditions of such pair of lamps is not substantially greater than said open circuit voltage of said output circuit, and a starting impedance connected across one of such lamps.

9. Alternating current power supply means for starting in sequence and operating in series a pair of fluorescent discharge lamps from a source of predetermined voltage and frequency comprising, a transformer having a core of magnetic material including a central leg having first and second portions and two outer legs, one each of which is disposed on each side of said central leg, the portion of said outer legs adjacent said second portion of said central leg having an increased cross-sectional area relative to the portion of said outer legs adjacent said first 17 portion of said central leg; a primary winding on said first portion of said central leg and adapted for connection to said source, a secondary winding on said second portion of said central leg adapted to be connected in circuit with such pair of fluorescent lamps, each of which has predetermined starting and operating voltages, and said portion of increased cross-sectional area of said outer legs and said first portion of said central leg being disposed adjacent each other to form a high reluctance magnetic shunt magnetically disposed between said primary and secondary windings and also between said first and said second portions of said central core leg, said primary and said secondary windings being connected in voltage additive auto-transformer relationship to form an output circuit and the ratio of turns of said primary winding and said secondary winding being such that the open circuit voltage of said output circuit when said primary winding is energized from said source is suflicient to start one of such lamps, said second portion of said central core leg having a reduced cross-sectional area relative to the crosssectional area of said first portion of said central leg, and a capacitor connected to one terminal of said output circuit and being adapted to be connected to one end terminal of one of such lamps, the other terminal of said output circuit being adapted to be connected to one end terminal of the other of such lamps, the other end terminals of each of such lamps being connected together, thereby defining a series operating circuit comprising said output circuit, said capacitor and such pair of lamps, when connected in circuit, said capacitor having a capacity value such as to provide substantially rated output in such pair of lamps, the reduction in cross-sectional area of said second portion of said central leg being such that the voltage across said capacitor during steady state operating conditions of such pair of lamps is not substantially greater than said open circuit voltage of said output circuit, and a starting impedance connected across one of such lamps.

References Cited in the file of this patent UNITED STATES PATENTS Foulke Oct. 6, 1936 Short July 16, 1946 Keiffer Oct. 4, 1955 

